1. Field of the Invention
The present invention relates to a reduced pressure processing apparatus and a method therefor, which are used in reduced pressure processing such as CVD processing for a semiconductor wafer.
2. Description of the Related Art
In the manufacture of a semiconductor device, appropriate processing such as CVD is performed to a semiconductor wafer. In low-pressure CVD processing, for example, evacuation is performed to change a pressure in a processing vessel purged with a nitrogen gas from an atmospheric pressure to a reduced pressure while the processing vessel is heated. In this case, when the processing vessel is rapidly evacuated with a high exhaust pressure from the beginning, the wafer is contaminated with particles raised in the processing vessel. For this reason, a main exhaust system having a high exhaust pressure and a sub-exhaust system having a low exhaust pressure are arranged. During evacuation, the processing vessel is slowly evacuated by using the sub-exhaust system with the low exhaust pressure first, and then the processing vessel is rapidly evacuated by using the main exhaust system with the high exhaust pressure.
Although an O-ring is used to air-tightly seal the processing vessel, the elastic deformation of the O-ring easily becomes insufficient due to its heat deterioration over time. For this reason, to evacuate the processing vessel with the low exhaust pressure, a force for pressing the O-ring is so weak that the O-ring cannot be brought into tight contact with a sealed portion. As a result, a gap may be formed around the O-ring.
As described above, when the processing vessel purged with a nitrogen gas is evacuated with a low exhaust pressure while the gap is formed around the O-ring, external air is drawn from the gap to result in entrance of an oxygen gas into the processing vessel. In this manner, when low-pressure CVD processing is performed while the oxygen gas is present in the processing vessel, an unnecessary spontaneous oxide film is formed on the surface of a silicon semiconductor wafer.
However, even when the oxygen gas is drawn into the processing vessel as described above, after the evacuation is performed by the sub-exhaust system, evacuation of the processing vessel with the high exhaust pressure may cause deformation of the O-ring by a shock to seal the openings around the O-ring. In this case, since there is no leakage any more, even when the pressure in the processing vessel to be tested is measured to determine whether leakage is present or absent, "no leakage" is obtained. That is, any leakage which has occurred in the evacuation using the sub-exhaust system with the low exhaust pressure cannot be detected.
As described above, in the conventional technique, even when an oxygen gas is drawn into the main exhaust system, this cannot be detected. For this reason, prior to the low-pressure CVD processing, a spontaneous oxide film having a considerably large volume may be formed on the surface of the silicon semiconductor wafer. Therefore, in this case, a film is to be formed on the spontaneous oxide film by the low-pressure CVD processing, and a production yield is considerably decreased.